Unlike conventional Cathode Ray Tubes (CRTs) whose color brightness level and therefore color intensity can be controlled by varying an analog brightness control voltage at the grid electrode of the tube while the electron beam is swept across different pixel positions of a display line, digitally controlled displays such as Liquid Crystal Display (LCD) lack an analog control electrode similar to the grid electrode of a CRT. For this reason, a number of techniques have been utilized to control the pixel color intensity in LCDs.
For Super Twisted Nematic (STN) LCDs (i.e., passive matrix LCDs), since only one-bit, which translates to 2 graylevels, is required for each color, Red, Green, and Blue, a total of eight display colors is possible. As such, a pixel brightness control technique known as the `frame-rate modulation` method is used to generate more gray-levels per color and therefore more number of colors in total for the display panel. Generally, in the frame-rate modulation method, the frequency of pixel energizing pulses sent to the power lines associated with the corresponding pixels is varied to control the color intensity. In other words, the color intensity (gray-level) depends on how often the pixel is turned on.
More particularly, in a traditional frame-rate modulation method, a mathematical formula is typically used to generate the frame-rate modulation data. With a mathematical formula, while some programmability and flexibility may be possible, the flexibility is rather limited. The reason is the range of frame rate modulation data is mathematically limited by the formula itself. Such limitation may in turn reduce the performance of the frame-rate modulation method. More specifically, since the levels of intensity available for each display color may be limited, the ability to prevent visual disturbances such as flickering may be reduced, etc.
Prior-art attempts to improve the performance of traditional frame-rate modulation methods include the approach of U.S. Pat. No. 5,185,602 wherein the energization of spatially adjacent pixels is scattered in time and pixels which are energized at the same time are spatially scattered to avoid the perception of visual disturbances such as flickering and movie marquee effect.
In U.S. Pat. No. 5,185,602, brightness-setting signals each having a brightness level associated with them are stored in a waveform memory. The brightness levels are assigned to predetermined areas of the display panel. The brightness levels are stored in an image memory and whose locations are identified by the display row and column numbers. A phase placement pattern (matrix) of D.times.D cells that corresponds to each brightness level is created to map the frame number that an individual pixel is to be energized. Accordingly, there are D frames associated with each phase placement pattern. In so doing, the energization of spatially adjacent pixels is scattered in time and pixels which are energized at the same time are spatially scattered to avoid the perception of visual disturbances. The phase placement patterns are predetermined to minimize visual disturbances. All the phase placement patterns are then stored in a pattern memory which as a result may be sizable.
Each cell in a phase placement pattern corresponds to a pixel and can be accessed by the row and column modulo-D based numbers, the frame number, and the brightness level. Next, the desired brightness-setting signal is retrieved from the waveform memory by the brightness level and its corresponding energized bit can be extracted using an bit position signal output from the pattern memory.
As demonstrated above, the method of U.S. Pat. No. 5,185,602 and its hardware implementation are complex and expensive to implement. At the same time, the flexibility afforded by it is somewhat limited because the frame-rate modulation data is essentially predetermined by the phase placement patterns. While some programming capabilities exist for varying the frame-rate modulation data, such variation can not be easily performed given the inherent characteristics and requirements of the phase placement patterns. As a result, under U.S. Pat. No. 5,185,602, the capability to adapt to different passive matrix LCD panels is limited.
Thus, a need exists for a frame-rate modulation apparatus and method that are simple, cost-effective, and can easily be adapted to different passive matrix LCD panels.